Process for the production of extra low
carbon stainless steel

ABSTRACT

THE PRODUCTION OF A LOW CARBON STAINLESS STELL WHEREIN A REDUCTANT IS INTRODUCED WITH IRON ORE SLAG INTO A LADLE, THE CONTENTS BEING RELADLED TO COMPLETE THE REACTION, SEPORATING THE STEEL FROM THE SLAG AND ADDING THE NECESSARY ALLOYING ELEMENTS WHICH YIELD A STAINLES STEEL.

United States Patent Olhce Reissued Jan. 19, 1971 27,037 PROCESS FOR THEPRODUCTION OF EXTRA LOW CARBON STAINLESS STEEL William Blelocb,Parktown, Johannesburg, Transvaal,

Republic of South Africa, assignor to Rand Mines Limited, Johannesburg,Transvaal, Republic of South Africa No Drawing. Original No. 3,300,302,dated Jan. 24, 1967, Ser. No. 355,895, Mar. 30, 1964. Application forreissue Dec. 10, 1968, Ser. No. 783,443 Claims priority, applicationRepublic of South Africa, Apr. 1, 1963, 63/1,393 Int. Cl. C22c 39/20,39/26, 39/44 U.S. Cl. 75--130.5 8 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE T he production of a low carbon stainlesssteel wherein a rednctant is introduced with iron ore slag into a ladle,the contents being reladled to complete the reaction, separating thesteel from. the slag and adding the necessary alloying elements whichyield a stainless steel.

This invention relates to the manufacture of steels which are alloys ofiron and chromium with a very low carbon content and more particularlyto the manufacture of stainless steel which may include further alloyingelements such as nickel, copper, molybdenum, vanadium and the like.

The proportion of carbon in such steels has a very great effect oncorrosion resistance, and also other desirable properties such asductility, weldability and deep drawing qualities, these being improvedthe lower the proportion of carbon in the steels. For this reason, ELCstainless steels (that is extra low carbon stainless steels) having amaximum carbon content of 0.03 percent are of great and rapidlyincreaing importance.

The present availability of ELC stainless steels is due entirely to theoxygen refining process, but the present very high cost is also due tofeatures inherent in the oxygen refining technique. Because carbon canbe oxidised and removed from chromium steel by oxygen refiningpreferentially to chromium it is possible by oxygen lancing or jettingto reduce the carbon content of molten chromium steel to 0.06 percentwith relative ease, and Without undue over-oxidation and loss ofchromium in the refining process. When oxygen refining is pressed beyondthis limit of 0.06 percent carbon, much more oxygen is consumed, thetemperature required is much higher, refractories life decreasessharply, the steel is heavily overoxidised, and there is an almost totalloss of chromium, and of vanadium if that element is present. The ELCsteel so obtained by oxygen blowing is difficult effectively tode-oxidise; its chromium content must be brought up to specificationalmost entirely by addition of the very expensive 0.03 percentlow-carbon ferrochromium free from tramp elements; and it is almostimpossible to achieve a significant vanadium content in the finishedsteel by addition of exceedingly expensive low-carbon ferro-vanadiumeven after the steel has been de-oxidised as far as possible withferro-silicon, without excessive loss of vanadium into the slag.

In the commercial manufacture of stainless steel sheets and otherarticles from the ingots there is a very large proportion of scrapproduced, for example, up to 50%, and for economic reasons this isnormally returned for reprocessing. For re-use, this revert scrap may bemelted in an arc furnace and during this operation picks up carbon fromthe electrodes so that even if the revert scrap is an ELC 18/8 stainlesssteel, for example, it still picks up carbon on melting and this excesscarbon is difficult to remove in the manufacture of a new batch of ELCstainless steel.

The object of this invention is to provide a simple and effective methodof obtaining a substantially carbon-free ferrous material for alloyinginto stainless steel to meet particular requirements and to includemodifications to this fundamental process to enable alloying elements tobe introduced in the most economic manner dependent on the particularcircumstances of the particular ores and other constituents availablefor the production of the stainless steel.

In accordance with this invention the method of production of ELCstainless steel comprises the simultaneous introduction into a ladle ofa solid crushed rcductant alloy of iron and silicon and a moltensuperheated basic iron ore slag, the ladle contents being thereafterreladled to bring the reaction to completion, separating the steel soformed from the slag and including in this steel the necessary alloyingelements to yield the stainless steel of desired composition.

Further features of this invention provide for the alloying elements tobe included in the final stainless steel by introduction of the elementsas their oxides or other suitable compounds in the basic iron ore slagor during manufacture of the silicon rcductant or by introducing theelements in a solid or molten state in the ladle reaction or byintroducing the elements into a remelted cast product from the ladlereaction or in any combination of these ways.

The invention also provides for the silicon reductant to have a siliconcontent of between about 45 and about percent by weight.

In a preferred form of this invention it is found that the iron ore slagis very conveniently produced from a Transvaal type titaniferous ironore having a vanadium pentoxide content. The complete operation for themanufacture of a nickel containing stainless steel may comprise fourstages as follows:

(1) Revert chromium-nickel stainless steel scrap, or otherchromium-nickel stainless steel scrap and ferrochromium, are convertedto an alloy of chromium, iron, nickel and silicon with a subordinateamount of aluminium, by introducing these metals or alloys into asubmerged arc furnace for producing the silicon alloy. If the stainlesssteel scrap is available it provides the source of the nickel in thesilicon alloy. If the nickel is not available from this source it may beobtained elsewhere. It is to be understood that the alloying elementsneed not essentially form part of the reductant but may be introducedinto the final stainless steel product in any of the ways set out in thespecification. The silicon alloy has a silicon content preferablybetween about 45 and about 75 percent, and it therefore has a very lowcarbon content, of the order of 0.04 percent or lower because ofdisplacement of carbon by silicon.

(2) A slag is made in a separate open-arc Heroult type furnace bymelting together Transvaal titaniferous iron ore containing 0.5% to 2%or higher of vanadium pentoxide the ore being crushed to minus 1 inch,or finer, and burned lime in the proportions lying between 4-0 to 100parts by weight of burned lime to 100 parts of the iron ore.

(3) The silicon alloy produced as in stage (i) is crushed to 100% minus/2 inch and reacted with the molten and superheated iron-ore lime slagmade as in stage (2) by simultaneous pouring of the molten slag and coldcrushed silicon alloy into a basic-lined or neutral lined ladle. Revertchromium-nickel ELC stainless steel scrap, other ELC stainless steelscrap, low carbon ferrochromium, nickel, other alloying elements, orsome or all of these metals, is or are added cold to the ladle prior tothe reaction, and/or during subsequent reladling to act as alloyingadditions in the correct proportions, and also to act as coolants toincrease the life of the ladle lining. The ladle is lined with magnesiteor other brick and rammed with magnesite or other basic rammingrefractory in its lower working part, and with a chromite or iron oreand sodium silicate ramming mixture in its upper working part, or withhigh alumina fire brick in its upper working part, or alternatively isentirely lined with basic ramming material.

(4) The contents of the reaction ladle are then reladled with one ormore pourings into other ladles and the final slag separted as far aspossible by top pouring. The contents of the final top pour ladle arethen transferred to a bottom-pour teeming ladle and the metal, 'which isnow the finished ELC stainless steel is finally teemed into ingot mouldsfor rolling.

The following specific example will clearly indicate the method ofcarrying out the invention.

Transvaal titaniferous iron ore containing 11.5 percent TiO 54.2 percentiron and 1.51 percent vanadium pentoxide was crushed to 100% minus 1inch and melted in a 1500 kva. Herouls type are furnace basic lined with70 parts by weight of burned lime containing 91.6 percent calcium oxide.3000 lbs. of the molten slag, superheated to approximately 1450 C., pourladle simultaneously with 640 lbs. of cold crushed (l0O% /2 inch)iron-chromium-silicon alloy having 14.1 percent iron, 26.2 percentchromium, 56.8 percent silicon, 0.032 percent carbon, the balance beingessentially aluminium. The reaction mixture was poured from a height ofabout 8 feet within a few minutes into a second ladle into which therehad previously been charged 160 lbs. of electrolytic nickel in the formof broken plates, and 150 lbs. of cold low carbon ferrochromiumcontaining 56.4 percent chromium and 0.028 percent carbon broken toapproximately fist-size lumps or smaller. The mixture was then re-ladledwithin a few minutes. from a similar height back into the first ladleinto which another 150 lbs. of the same ferrochromium had been placed.The mixture of slag and metal was then repoured twice from one ladle tothe: other, the final pouring being made into a bottom-pour ladle afterdiscarding about two thirds of the slag to a wet slag-granulator, andthe finished ELC chromium-nickel stainless steel was teemed into 500lbs.

ingot moulds and the steel recovered as ingot. The

analysis of the finished steel was 0.024 percent carbon, 17.3 percentchromium, 9.8 percent nickel, 0.023 percent sulphur, 0.012 percentphosphorous, 0.62 percent vanadium and 0.24 percent manganese, thebalance being mainly iron.

The above example illustrates a particular case where the alloyingelement vanadium was introduced through its oxide in the iron ore slag,chromium through its inclusion during the production of the reductantand with nickel in the ladle reaction.

The example shows the use of solid alloying elements in the reactionladle but under some circumstances it may be desirable to introduce themin a molten state.

The product obtained in any of the manners referred to above and bygenerally following the four steps of production set out need not beteemed into ingot moulds for final processing but may be cast to formstock base metal for remelting and alloying with the necessary elementsto give a stainless steel meeting specified requirements.

The product of this remelting will be either teemed into required ingotsize or cast continuously as may be required.

To illustrate this latter modification to the process an example isgiven using a steel obtained by the method generally set out above.

was poured into a top- In this example the steel produced as describedis used as stock material for remelting and had the followingcomposition by weight:

Percent The steel was remelted in a high frequency induction furnacehaving a silica lining. The steel was then further alloyed with nickel,chromium, manganese and copper and cast into 400 lbs. ingots by bottompouring from a one inch diameter nozzle via a hot-top of conventionaldesign.

The finished ingot gave a final percentage analysis as follows:

Percent Si 0.38 S 0.02

P (by weight) 0.01

It will be appreciated that the alloying elements may be introduced intothe final steel in a variety of combinations using the fundamentaltechnique of reducing a basic iron ore slag to a steel of negligiblecarbon content.

It will also be appreciated that the rate of reaction between the basiciron ore slag and the reductant as hereinbefore described, may bemodified by the introduction of. suitable diluent metals and/or slagforming materials in the ladle or with the reductant.

What I claim as new and desire to secure by Letters Patent is:

1. A process for the production of extra low carbon stainless steelcomprising the reaction of a reductant with an iron ore slag by thesimultaneous introduction into a ladle of a [solid crushed] siliconreductant alloy having a silicon content of between about 45 percent toabout 75 percent by weight and having a carbon content of less thanapproximately 0.04% carbon and a molten superheated basic iron ore limeslag comprising lime in the amount of at least .4 by weight of the ironore, the ladle contents being thereafter reladled to bring the reactionto completion, separating the steel so formed from the slag andincluding in this steel the necessary alloying elements to yield astainless steel of desired composition.

2. A process as claimed in claim 1 in which at least one of the alloyingeelments is introduced into the final stainless steel by its inclusionas oxides in the iron ore slag, said iron ore slag containing 0.5percent to 2 percent of the alloying element vanadium pentoxide.

3. A process as claimed in claim 1 in which at least one of the alloyingelements is included in the silicon reductant during preparation of thelatter.

4. A process as claimed in claim 1 in which at least one of the alloyingelements is introduced by insertion into the reaction ladle prior to thesimultaneous pouring of slag and reductant.

5. A process as claimed in claim 1 in which at least one of the alloyingelements is introduced by insertion into the reaction ladle during thereaction process.

6. Process of producing extra low carbon stainless steel comprisingproducing a silicon alloy wherein the silicon content is between 45% and75% by melting chromiumnickel stainless steel scrap, iron, silicon andferro-chromium and allowing the molten product to solidify; producingslag by melting together iron ore containing .5% to 2% of vanadiumpentoxide and burned lime, the lime being in the amount of at leastapproximately 40% by weight of the iron ore; simultaneously introducingsaid silicon alloy in ,cold crushed form and said iron-ore lime slag inmolten state in a reaction ladle; reladling the contents of saidreaction ladle; and separating the slag from the molten metal by toppouring.

7. The process of claim 1 wherein the reduclant allay is introduced intothe ladle in a molten state.

8. The process of claim ,I, wherein the reductant alloy is introducedinto the ladle in a solid crushed state.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

HYLAND 6 UNITED STATES PATENTS 3/1933 Custafesson 75130.5X 11/1937Perrin 7524 11/1937 Perrin 75130.5X 11/1948 Loveless 75-l 30.5X 11/1948Loveless 75130.5X 7/1962 Udy et a1. 7530X 1/1963 Kuhlmann 7530X 9/ 1964Mennenoh 753( X BIZOT, Primary Examiner I. E. LEGRU, Assistant ExaminerUS. Cl. X.R.

